Free flow micropuncture studies have been designed to systematically evaluate uric acid transport along the length of the rat nephron. Fractional uric acid net transport will be assessed in the early and late proximal tubule and early distal tubule during the infusion of (2-14C) uric acid and (methoxy-3H) inulin. Net transport in the early proximal tubule, between the early and late proximal tubular sites and loop of Henle will be evaluated. Comparison of net transport at the early distal tubule and fractional uric acid excretion will provide information on the characteristics of net transport within the distal tubule and collecting duct. Studies will be performed in hydropenic rats to further assess the role of sodium and water balance on net uric acid transport, in mannitol loaded animals to define the effects of this agent on segmental uric acid transport and in uric acid loaded, mannitol infused animals to evaluate the effects of acute hyperuricemia on the characteristics of uric acid transport along the nephron.